The present invention relates to radio frequency (RF) coils and preamplifiers used in magnetic resonance (MR) applications. It finds particular application to MR imaging applications in which RF receive coils having relatively high quality factors (Qs) are used.
MR imaging has proven to be a valuable technique for providing information about the internal structure and function of an object under examination. In medical imaging, for example, MR imaging techniques are widely used to provide information on the physiology of human patients.
One limitation, however, on the utility of images and other information generated by MR scanners is the effect of noise. Indeed, signal to noise ratio (SNR) is a key parameter used to evaluate the quality of the information generated by an MR system.
Various techniques have been used to improve MR system SNR. One technique for improving SNR has been the use of low noise RF receive coils and preamplifiers. SNR is enhanced if the noise introduced by the receive coil and preamplifier is small in relation to the noise introduced by the object under examination (e.g., tissue noise in the case of a human patient).
Low noise receive coils have been implemented, for example incorporating high temperature superconductor (HTS) material or cold copper (i.e., conventional copper coils cooled to liquid nitrogen temperatures) to reduce the coil resistance. While these coils provide improved noise performance, they are characterized by coil Qs which are relatively higher than those of conventional coils. The higher Qs, however, lead to decreased coil bandwidth. Bandwidth is typically expressed as the ratio of coil resonant frequency divided by the coil Q. Thus, for example, a coil having a resonant frequency of 8.6 Megahertz (MHz) and a Q of 3300 would have a bandwidth of 2.6 kilohertz (KHz).
Moreover, higher Q coils are relatively more sensitive to the effects of coil loading, which can be subject to significant inter-patient or inter-object variability, and can also be affected by object motion. These effects tend to alter the phase response and impedance of the coil. Changes in capacitive loading, thermal effects, and the like can also cause shifts in coil resonant frequency. As coil bandwidth decreases, these shifts become relatively more significant.
Other trends, such as the development of faster pulse sequences, more powerful gradient systems, and MR guided interventional procedures in MR imaging have, on the other hand, placed increasing demands on coil and receive system bandwidth. In some cases, the bandwidth required by the MR system can greatly exceed the coil bandwidth. In any case, system performance can be enhanced by reducing the noise contribution of the coil while maintaining a reasonable coil bandwidth.
Various techniques have been used to expand coil bandwidth, for example as disclosed in U.S. Pat. No. 5,051,700 to Fox entitled Feedback Circuit for Noiseless Damping of the Q of an MRI Receiver Coil Antenna, U.S. Pat. No. 5,488,382 to Fenzi entitled Low Noise Preamplifier, and Chang, et al., Stability and Noise Performance of Constant Transimpedance Amplifier with Inductive Source, IEEE Transactions on Circuits and Systems, Vol. 36, No. 2, pp. 264-271 (February 1989).
Another technique has involved the use of double-tuned receive coils. However, double tuning has limited applicability. Broad bandwidth requires that the preamplifier noise remain low relative to the detector noise of a wide bandwidth, and that the power gain be high over this bandwidth. Single or multi-tuned coils maximize the low noise bandwidth by ensuring that the coil impedance alternates between two extremes: a high impedance limit that does not exceed the preamplifier current noise, and a low resistance limit that remains above the preamplifier voltage noise. Double coupling is one example of this technique, more generally called overcoupling. Unfortunately, however, overcoupling also results in relative large swings in signal amplitude over the effective bandwidth.